The invention relates generally to screw drive mechanisms and more particularly, to a new and improved mechanism for use in pumps, such as a syringe pump, requiring the engagement and disengagement with a lead screw for use in coupling the pump to a device for the administration of medical fluids.
In the field of syringe pumps which are used for dispensing medical fluids at an accurate rate and in some cases, at a slow and accurate rate, some designs have used lead screws. Typically, a nut or traveler mechanism is engaged with the lead screw and moves along the lead screw in accordance with the rotation thereof. Coupled to the traveler mechanism and moving therewith is typically an arm which is also coupled to the plunger of the syringe to apply pressure to the plunger which in turn applies pressure to the fluid contained in the syringe for the delivery thereof.
In many cases, self-contained syringes are used. These syringes are pre-loaded with varying amounts of fluid and the syringe plunger may be in any position. The syringe pump arm must be movable along the lead screw to accommodate these preloaded syringes. One prior technique for positioning the arm used the rotation of the lead screw itself to position the arm prior to syringe installation. Because such a technique is based on trial-and-error for correct placement, it could be very time consuming.
Another prior approach employs a traveler mechanism having a half-nut mounted on the lead screw. Upon loading a new syringe, the half-nut is moved away from the lad screw so as to disengage from the lead screw threads, moved to the proper position and moved back into engagement with the lead screw. Various techniques have been developed for assuring that the half-nut of the traveler mechanism remains in contact with the lead screw during the engaged mode. In the cases of tightly fitting syringe plungers or viscous fluids in the syringe and which require considerable force to move the plunger, the half-nut must be held against the lead screw with sufficient force so that undesired disengagement does not occur during the development of the force required to pump the syringe contents. In one prior approach, a spring was used to exert considerable pressure on the half-nut to retain its contact with the lead screw threads. This pressure can have the effects of excessive lead screw and half-nut thread wear as well as deformation of the lead screw, such as bowing.
Another problem encountered with prior half-nut techniques is the transmission and in some cases the amplification of the imperfections in the lead screw to the arm mechanism. For example, lead screws having threads following a varying helix or which are otherwise non-uniform around the screw will cause a "wobbling" of the traveler mechanism. This wobbling can be amplified by the moment of the arm which is coupled to the syringe plunger. Such wobbling may cause uneven pressure on the fluid in the syringe which results in flow variations and inaccuracy. While it may be possible to manufacture more accurate lead screws, their cost would be higher.
Hence, it would be an advantage to provide a traveler mechanism which is disengagable from the lead screw threads so that it may be positioned along the lead screw as desired, which does not require a large spring to provide force to retain the contact between the half-nut and the lead screw, and which provides a means of decoupling undesired motion of the lead screw from the arm so that transmission of lead screw imperfections to the arm is reduced.